Electromagnetic compatibility (EMC) pertains to the capability of an electronic device or system to operate correctly within its electromagnetic surroundings without causing unacceptable interference to other devices in the vicinity. EMC encompasses both electromagnetic interference (EMI) and electromagnetic susceptibility (EMS). EMI involves external disruptions affecting electrical products, while EMS refers to the resilience of electrical products against such disturbances. An optimally compatible device should remain unaffected by surrounding electromagnetic noise and avoid generating undue interference itself. The three critical components of EMI are the source of interference, the coupling medium, and the susceptible entity. Minimizing the interference produced by switching power supplies is crucial for maintaining the seamless and stable functioning of electronic systems. Electromagnetic interference suppression techniques primarily focus on reducing the intensity of the interference, interrupting or weakening the noise propagation pathways, and enhancing the equipment’s resistance to electromagnetic disturbances.
**1. Causes of Electromagnetic Interference in Switching Power Supplies**
Switching power supplies typically convert commercial alternating current into direct current, then transform it into high-frequency currents via controlled switching transistors, and finally deliver stable direct current voltage through rectifiers and filters. During this process, the high-capacity capacitors used in power frequency rectification and filtering undergo rapid charge-discharge cycles, while the switching tubes and output rectifier diodes experience frequent high-speed switching and reverse recovery. These actions generate significant di/dt (rate of change of current) and du/dt (rate of change of voltage), creating intense inrush currents and spike voltages. These phenomena represent the fundamental origins of electromagnetic interference in switching power supplies. Additionally, the driving waveforms of switching transistors, particularly those of MOSFETs, often resemble rectangular waves at megahertz frequencies. Such high-frequency signals can disrupt the essential operations of the switching power supply, particularly those within the control circuitry.
**1.1 Input Harmonic Interference from the Rectifier Circuit**
The input stage of a switching power supply commonly employs a bridge rectifier and capacitor filter configuration. The rectifier bridge only conducts when the ripple voltage surpasses the voltage across the input filter capacitor. As the mains voltage charges the filter capacitor, the rectifier remains active until the capacitor's voltage exceeds the instantaneous mains voltage, at which point it shuts off. Consequently, the input circuit experiences pulsating currents with a high proportion of harmonic content, a consequence of the rectifier circuit’s non-linear behavior. The AC-side current becomes significantly distorted as a result.
On the DC side, the harmonic content is n-fold the input frequency. Thus, the high-frequency harmonic currents on the rectifier circuit’s DC side not only lead to inefficiencies but also increase reactive power. These high-frequency harmonics can propagate along the transmission lines, inducing conducted and radiated interference.
**1.2 Interference Generated by the Switching Circuit**
The switching circuit serves as a pivotal element in the switching power supply and is a major contributor to electromagnetic interference. The load of the switch tube is the primary coil of the high-frequency transformer, which acts as an inductive load. When the switch tube conducts, a large inrush current flows through the primary coil, producing a high surge spike voltage across its terminals. Conversely, during the off-state, some energy from the primary coil may not fully transfer to the secondary coil due to leakage flux. The stored energy in the inductor combines with the capacitor and resistor in the collector circuit to form a damped oscillation peak,å åŠ onto the turn-off voltage to create a voltage spike. If this spike reaches a sufficiently high amplitude, it could potentially damage the switch.
Further research into these mechanisms is essential for developing effective solutions to mitigate interference and enhance the reliability of switching power supplies.
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